Recent evidence suggests that oxygen levels were suppressed worldwide 175
million to 275 million years ago and fell to precipitously low levels
compared with today’s atmosphere, low enough to make breathing the air at
sea level feel like respiration at high altitude.

Now, a University of Washington paleontologist theorizes that low oxygen and
repeated short but substantial temperature increases because of greenhouse
warming sparked two major mass-extinction events, one of which eradicated 90
percent of all species on Earth.

In addition, Peter Ward, a UW professor of biology and Earth and space
sciences, believes the conditions spurred the development of an unusual
breathing system in some dinosaurs, a group called Saurischian dinosaurs
that includes the gigantic brontosaurus. Rather than having a diaphragm to
force air in and out of lungs, the Saurischians had lungs attached to a
series of thin-walled air sacs that appear to have functioned something like
bellows to move air through the body.

Ward, working with UW biologist Raymond Huey and UW radiologist Kevin
Conley, believes that breathing system, still found in today’s birds, made
the Saurischian dinosaurs better equipped than mammals to survive the harsh
conditions in which oxygen content of air at the Earth’s surface was only
about half of today’s 21 percent.

"The literature always said that the reason birds had sacs was so they could
breathe when they fly. But I don’t know of any brontosaurus that could fly,"
Ward said. "However, when we considered that birds fly at altitudes where
oxygen is significantly lower, we finally put it all together with the fact
that the oxygen level at the surface was only 10 percent to 11 percent at
the time the dinosaurs evolved.

"That’s the same as trying to breathe at 14,000 feet. If you’ve ever been at
14,000 feet, you know it’s not easy to breathe," he said.

Ward believes low oxygen and greenhouse conditions caused by high levels of
methane from intense volcanic activity are likely culprits in mass
extinctions that occurred about 250 million years ago, at the boundary
between the Permian and Triassic periods, and about 200 million years ago,
at the boundary between the Triassic and Jurassic periods. He will make a
presentation on the topic Tuesday at the American Geological Society annual
meeting in Seattle.

The Permian-Triassic extinction is believed to have eradicated 90 percent of
all species, including most protomammals, a group of mammal-like reptiles
that were the immediate ancestors of true mammals. The Triassic-Jurassic
extinction killed more than half the species on Earth, with mammal-like
reptiles and true mammals, which evolved during the Triassic Period, hit
particularly hard. But dinosaurs, which also evolved between the two
extinctions, had little problem with conditions during the Triassic-Jurassic

"The seminal observation is that dinosaurs skated across the second of these
mass extinctions, actually increasing in number as they went along, while
everything else was dropping around them," Ward said.

Scientists know of five mass extinction events in Earth’s history, but a
cause has been widely agreed upon for only one – the episode at the end of
the Cretaceous Period 65 million years ago, when the impact of an asteroid
is believed to have brought the demise of the dinosaurs. Such impact also
has been suggested as the cause of the Permian-Triassic and
Triassic-Jurassic extinctions, but geologists have yet to unearth any
indisputable evidence of such an impact, and there is no conclusive evidence
of what caused either of the events.

Ward said mass spectrometer readings on fossil material, as well as the
extinction pattern for fossils in rock outcrops collected from the time of
the two extinctions, indicates the events were drawn-out affairs and did not
happen suddenly, as they would have with an asteroid impact.

In addition, he said it is known which types of creatures, and which
breathing systems, best survived the extinction events. The same breathing
systems are still present in birds, which are known to fare well at high
altitudes, where oxygen levels are substantially lower than at the surface.

"The reason the birds developed these systems is that they arose from
dinosaurs halfway through the Jurassic Period. They are how the dinosaurs
survived," he said.